1. Field of the Invention
This invention relates to detecting circuits and, more particularly, to the detection of a direct current voltage in the presence of interfering alternating current and direct current voltages.
2. Description of the Prior Art
It is well-known to provide the detection of direct current voltages by means of voltage threshold devices which respond to voltages exceeding that threshold. Particular problems arise, however, in environments in which spurious voltages are also present and which must be discriminated against when detecting the desired voltage. Thus, direct current voltages of different polarity or of different magnitudes must be discriminated against as well alternating current voltages which may have components exceeding the test voltage. One such system is shown in U.S. Pat. No. 4,270,030 issued May 26, 1981 to S. J. Brolin et al.
In a telephone system environment, it is often necessary to detect a particular line voltage level while, at the same time, not to interfere with standard test responses such as leakage tests and foreign potential tests. The detector circuit must therefore not only respond correctly to the desired voltage, but also present a high enough impedance to permit standard leakage tests. In addition, the detector circuit must not feed voltages back into the telephone circuit which would cause false responses during a foreign potential test.